How to discover default values of properties via python script?
To get all the default values of all properties defined on the context object.  Using the RNA (bl_rna) will apply to all objects in blender.

import bpy

context = bpy.context
obj = context.object

for prop in obj.bl_rna.properties:
    if hasattr(prop, "default"):
        default = prop.default
        print("%s (%s) default:" % (prop.identifier, prop.name), prop.default)
        # check arrays
        if getattr(prop, "is_array", False):
            default_array = [p for p in prop.default_array]
            print("default array", default_array)
            current_value = [p for p in getattr(obj, prop.identifier)]
            print("current value", current_value)
            print("is default value", current_value == default_array)


Also have a look at 2.78/scripts/modules/rna_xml.py  which creates xml from blender objects.